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Specialists

Isn’t it strange that a plant that contains so many nasty chemicals (e.g., cardiac glycosides), as well as rubbery latex so alkaline that it can permanently scar the cornea of one’s eye, has so many insects that specialize on it?

But here they are — the amazing milkweed fauna: lepidopterans, bugs, and beetles, consuming every part of the milkweed plant from its roots to its seeds — all seen in the backyard this summer.

The familiar Monarch butterfly caterpillar, munches away happily undeterred by the milky latex exuding from the leaves and stems of the plant.

The less familiar Milkweed Tussock Moth larvae — there were so many caterpillars on this particular milkweed plant, they completely defoliated it.

The tussock moth larvae grows some very long tufts and is not quite so gregarious when it’s older.

Milkweed bugs (true bugs — Hemiptera) are usually found on milkweed plants that have formed seed pods. They lay a clutch of bright yellow eggs on one of the pods, and the nymphs develop through five molts into adults by feeding through the pod wall on the seed endosperm.

Yellow aphids collect on milkweed stems and pods, but feed on the sugars passing through the plant’s phloem vessels, not the seeds. Small wasps (left center) parasitize the aphids by laying their eggs on the host. Aphids are actually true bugs (Hemiptera), although these non-winged individuals don’t appear very bug-like.

The Red Milkweed Beetle is a member of the long-horned beetle family. They lay their eggs near the ground, and the larvae burrow into the roots and develop and overwinter there to emerge as adults the following spring. Like the monarch butterfly larvae, milkweed beetles incorporate the milkweed’s poisonous chemicals into their own bodies, becoming distasteful to their predators.

Milkweed leaf beetles are members of the very large leaf-beetle family. They eat the leafy greenery, but the larvae are also known for consuming each other — their cannibalistic tendencies reduce competition for food in their local area!

Isn’t it ironic that in producing poisons to ward off herbivores, the plant becomes more attractive to specialist herbivores also trying to avoid predation?

Would it have anything to do with how long it has been around for ? Plants are so interesting. Do you anything about why different varieties of the same flower species can have a nectar that is more attractive to pollinating insects? Amelia

If it is the same species, then I would guess that there has been some selection for nectar production, either directly by selecting for which pollinators visit it, or indirectly with nectar production going along with modifications of floral structure (e.g., flower size or color). Are you thinking of a particular plant species in which this occurs–that might make a difference in my answer.

Well, actually that’s a good question. So is it better to be a generalist plant species that a lot of different herbivores might take a bite of, or a specialist plant that only a few select herbivore species can tolerate but on which large populations (of aphids or milkweed bugs, for example) might develop? It’s probably a no-win situation.